OP, the main thing you want to do is
find the forum for the phone that you have. Then look through and see what ROM's are available, and what features/benefits the ROMs have, and decide if that's a ROM you'd like to have on your phone. A lot of them come with custom tweeks and optimizations that really make your phone perform better.
Then you need to root your phone... generally speaking the forum for your phone should have an "all things root" subforum where you can find information on how to root your phone. Be sure to read everything carefully until you fully understand the process, and by all means if you have questions don't hesitate to ask BEFORE doing anything so that you're prepared.
Then once you've successfully rooted your phone, you can find the ROM you want, then read that thread completely (all the posts, not just the OP) so that you know all about the ROM, how to install, and other peoples posts about the ROM. Once you're clear on how to do everything properly, follow the instructions on how to install
You also may want to check
xda developers for more ROM's as not all of them that are available are posted here. There's a lot of awesome ROM's available, the developers put in a lot of time and hard work on them.